fix(discover): drop suggestions for non-artists (no core discography)

ListenBrainz surfaces producers/session players/band members (Max Martin,
Dominic Howard) that co-occur in listens but have no releases of their own —
they showed as bare rows with an empty page. _derive_albums now drops a
surfaced artist (deletes their pending suggestion) when their MB discography
has no core release-group (Album/Single/EP, no secondary type), catching both
empty discographies and soundtrack/production-only credits, while keeping real
singles/EP-only artists. On browse failure the suggestion is kept.

Test fake browser gains default_core so scoring tests keep surfacing candidates.
worker 249 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/lyra_worker/discovery.py

@@ -204,6 +204,16 @@ def _derive_albums(conn: psycopg.Connection, browser: MbBrowser, surfaced: list[
         except Exception as e:  # one artist's browse failure must not abort the sweep
             print(f"worker: discovery album browse failed for {artist['mbid']}: {e}", flush=True)
             continue
+        # Drop suggestions for "artists" with no real discography of their own — producers,
+        # session players, band members, soundtrack-only credits. ListenBrainz surfaces them
+        # (they co-occur in listens) but they have no core release-group to follow or want, and
+        # their page is empty. Require at least one core release (Album/Single/EP, no secondary
+        # type); on browse failure we keep the suggestion rather than drop a real artist.
+        if not any(is_core_release(g.primary_type, g.secondary_types) for g in groups):
+            with conn.cursor() as cur:
+                cur.execute("DELETE FROM \"DiscoverySuggestion\" WHERE status = 'pending' "
+                            'AND "artistMbid" = %s', (artist["mbid"],))
+            continue
         core = [g for g in groups
                 if _album_kind_ok(g, cfg.albums_only) and g.rg_mbid not in have]
         core.sort(key=lambda g: g.first_release_date or "", reverse=True)